Geomembrane Wedge Welding

Wedge welding is the primary welding technique employed by Layfield in the field and in the shop.

One of Layfield’s core competencies is the welding of thermoplastic sheets in field construction. One of the techniques used by Layfield is thermal wedge welding.

Wedge welding machines use a heated element to fuse thermoplastic sheet materials. Wedge welders are automatic machines that run between two overlapped sheets of material. The welder heats the interface between the materials and then presses the materials together to form a lap weld. The thermoplastic materials are fused at the molecular level without the addition of any glues or solvents.

Each brand of welder may have a slightly different arrangement of wedges, overlaps, and pressure rollers. Layfield has a number of different wedge welder types to address any welding situation.

Wedge welding is most commonly performed on HDPE sheet materials, however Layfield has the skills to weld most thermoplastic geomembrane materials. One of Layfield’s unique skills is the ability to field wedge weld PVC materials in any thickness. Layfield has also developed thin film welding techniques using specialized equipment. With these special welders Layfield can routinely weld sheeting as thin as 0.5 mm (20mil). Nonwoven geotextiles can also be welded with wedge welding techniques, however this type of welding only tacks the geotextile together. For better geotextile seams Layfield provides field and shop geotextile sewing services.

Note1: T refers to a textured geomembrane material (one side or two sided).
Note2: S refers to a supported fabric scrim in the material.

Layfield uses wedge welding as the basic technique for production welding in field installations. Wedge welders run at a continuous speed (dependent on the material) between clean, overlapped material. Wedge welders are used exclusively for straight run welds. A wedge welder cannot be used for detail work or curves. A wedge welded job will almost always require some finishing work. For finishing work please see the Layfield tech notes on the extrusion welding and bonded seam welding.

There are two basic wedge arrangements, the solid wedge, and the split wedge. The solid wedge is used for thin film materials and produces a fully bonded weld not less than 25 mm (1 inch) in width. Solid wedge welds are normally tested with an air lance test. The split wedge is more commonly used on thicker materials. The split wedge welds two tracks with an unbonded channel between them. At the completion of a weld this channel is sealed at both ends and pressurized with air. This air pressure test provides a non-destructive indication of seam integrity and continuity. For details on air lance and air pressure testing please see the Layfield tech note on testing.

Wedge welds are also routinely tested using destructive testing. Layfield’s crews are equipped with field portable tensiometers for field destructive testing. Wedge welds are tested in shear and in peel. Shear testing is the basic test of strength across a weld while peel testing is used to confirm weld quality. Peel tests are usually performed on both sides of a wedge weld (usually called left and right peel). Specifications for shear and peel tests are contained in the geomembrane specification booklet and in the section in the HDPE specification section.

Field destructive testing results can vary based on the ambient temperature. Results in cold weather may be very high, while test results in hot weather may be below specification. Layfield has developed adjustments for test results from ambient temperature to standard test temperature. The acceptance of all test results will be based on a standard test temperature of 23°C (73.4°F) and not ambient temperature.

Field welding requires that the surfaces to be joined are clean and dry. There can also be limits due to temperature and other environmental conditions. Wedge welders are especially sensitive to the presence of moisture such as rain, dew, snow, frost, or standing water. Cold weather installation is limited by materials and moisture conditions rather than temperature. Please see the Layfield tech note on cold temperature installations for more information.

Wedge welders are set up at the beginning of each job. Adjustments are made for material and site conditions. Trial strips, called qualification welds, are prepared and tested at site to confirm that all setting are correct. Welding will not begin until a qualification weld is prepared that meets project specifications.

The geomembrane to be welded is prepared for welding by overlapping a set distance and cleaning the overlap area. The overlap required for wedge welding is machine dependant and varies from 50 to 150 mm (2 to 6 inches). It is the overlap area that is going to be welded so cleanliness in this area is essential. Cleaning is usually done with water and rags immediately prior to welding. The welder operator will inspect the overlap area for cleanliness and moisture conditions ahead of the welder.

The subgrade for wedge welding should be clean with no rocks, ruts, or other surface imperfections. The bottom wheels of the welder run on the subgrade so moisture, mud, or other deletrious materials can stick to the bottom wheel and cause hang-ups. A slip sheet is a narrow strip of material that is placed under the weld area on which the bottom wedge welder wheels can run. The slip sheet may be moved after welding or left in place at the contractor’s option. Placing a geotextile immediately under the geomembrane has the double benefit of providing a clean working surface for wedge welding and providing protection of the geomembrane from subgrade abrasion.

If you have any questions regarding wedge welding or field installations please contact your Layfield representative.